1. Technical Field
The present invention relates generally to high-performance, fault-tolerant HTTP, streaming media and applications delivery in a content delivery network (CDN).
2. Description of the Related Art
It is well-known to deliver HTTP and streaming media using a content delivery network (CDN). A CDN is a self-organizing network of geographically distributed content delivery nodes that are arranged for efficient delivery of digital content (e.g., Web content, streaming media and applications) on behalf of third party content providers. A request from a requesting end user for given content is directed to a “best” replica, where “best” usually means that the item is served to the client quickly compared to the time it would take to fetch it from the content provider origin server. An entity that provides a CDN is sometimes referred to as a content delivery network service provider or CDNSP.
Typically, a CDN is implemented as a combination of a content delivery infrastructure, a request-routing mechanism, and a distribution infrastructure. The content delivery infrastructure usually comprises a set of “surrogate” origin servers that are located at strategic locations (e.g., Internet Points of Presence, access points, and the like) for delivering copies of content to requesting end users. The request-routing mechanism allocates servers in the content delivery infrastructure to requesting clients in a way that, for web content delivery, minimizes a given client's response time and, for streaming media delivery, provides for the highest quality. The distribution infrastructure consists of on-demand or push-based mechanisms that move content from the origin server to the surrogates. An effective CDN serves frequently-accessed content from a surrogate that is optimal for a given requesting client. In a typical CDN, a single service provider operates the request-routers, the surrogates, and the content distributors. In addition, that service provider establishes business relationships with content publishers and acts on behalf of their origin server sites to provide a distributed delivery system. A well-known commercial CDN service that provides web content and media streaming is provided by Akamai Technologies, Inc. of Cambridge, Mass.
CDNSPs may use content modification to tag content provider content for delivery. Content modification enables a content provider to take direct control over request-routing without the need for specific switching devices or directory services between the requesting clients and the origin server. Typically, content objects are made up of a basic structure that includes references to additional, embedded content objects. Most web pages, for example, consist of an HTML document that contains plain text together with some embedded objects, such as .gif or jpg images. The embedded objects are referenced using embedded HTML directives, e.g., Uniform Resource Identifiers (URIs). A similar scheme is used for some types of streaming content which, for example, may be embedded within an SMIL document. Embedded HTML or SMIL directives tell the client to fetch embedded objects from the origin server. Using a CDN content modification scheme, a content provider can modify references to embedded objects so that the client is told to fetch an embedded object from the best surrogate (instead of from the origin server).
In operation, when a client makes a request for an object that is being served from the CDN, an optimal or “best” edge-based content server is identified. The client browser then makes a request for the content from that server. When the requested object is not available from the identified server, the object may be retrieved from another CDN content server or, failing that, from the origin server.
In some CDNs, such as Akamai FreeFlow® content delivery service, data about the content provider's (CP's) objects, or so-called “metadata,” is often directly encoded “in-URL,” namely in the HTML or SMIL directives that are modified during the content modification process. More specifically, metadata is the set of all control options and parameters that determine how a CDN content server will handle a request for an object. Such metadata may include, for example, a CP code or other internal tracking number used, for example, to facilitate billing, coherence information (e.g., TTL or fingerprint) about how CDN servers should cache the object and maintain its freshness, a unique serial number value that may be used for load balancing, access control data, a hostname identifying the origin server where a copy of the object may be located, and other feature-specific metadata.
By including object metadata directly in the HTML or SMIL directives, content providers may set up their metadata as part of the publication process, i.e., without requiring changes in their web server or involving network operations personnel. The “in-URL” embedding technique ensures that any modified URL pointing to the CDN has, in a self-contained way, the information needed to serve the object. On the other hand, the modified URL generated by this process is often long and complex. In addition, some content providers may only have a need to specify site-wide or global metadata specifications.
Thus, there remains a need to provide a framework that allows for both a simple method of creating modified URLs for sites with simple global metadata specifications, while allowing arbitrary complexity for sites with arbitrarily complex metadata needs.